Remote replication of snapshots taken while replication was inactive

ABSTRACT

Snapshots of a first logical storage unit (LSU) (R1) on a first storage system (A1) may be taken while replication is inactive between A1 and a second storage system (A2), such that these outstanding snapshots are not replicated to a second replica LSU (R2) on A2. Upon replication becoming active, the outstanding snapshots may be replicated to R2 without disrupting or impairing the remote replication of write operations between R1 and R2 as part of standard remote replication. A process on A1 executing the replication of the outstanding snapshots from R1 to R2 may be a separate process than one or more processes on A1 executing standard remote replication including the replication of write operations from R1 to R2. The process may be given low priority on A1 so as to not impair performance of other operations, including standard remote replication and replicating outstanding write operations on A1.

BACKGROUND Technical Field

This application generally relates to data storage networks, and more particularly to remotely replicating snapshots from one storage system to another storage system on a storage network.

Description of Related Art

Data storage systems (often referred to herein simply as “storage systems”) may include storage resources used by one or more host systems (sometimes referred to herein as “hosts”), i.e., servers, to store data. One or more storage systems and one or more host systems may be interconnected by one or more network components, for example, as part of a switching fabric, to form a data storage network (often referred to herein simply as “storage network”). Storage systems may provide a variety of data services to host systems of the storage network.

A host system may have host applications that utilize the data services provided by one or more storage systems of the storage network to store data on the physical storage devices (e.g., tape, disks or solid state devices) thereof. For a given application, to perform input/output (I/O) operations utilizing a physical storage device of the storage system, one or more components of the host system, storage system and network components therebetween may be used. The one or more combinations of components of the host, switching fabric and storage system over which I/O operations between an application and the storage system may be communicated may be considered an I/O path between the application and the storage system. It should be appreciated that other combinations of components of a storage network, for example, two or more storage systems, also may be coupled together by one or more switches of a switching fabric. Thus, more generically, the one or more combinations of components of a first network component, switching fabric and second network component over which I/O communications may be communicated may be considered an I/O path between the two network components. The collective I/O paths between components of a storage network may be considered to define a connectivity of the storage network.

Host systems may not address the physical storage devices of a storage systems directly, but rather access to data may be provided to one or more host systems from what the host system(s) view as a plurality of logical storage units (LSUs) including, for example, logical blocks, logical devices (also referred to as logical volumes, LUNs and logical disks), thin devices, groups of logical devices (e.g., storage groups), NVMe namespaces, and other types of LSUs. LSUs are described in more detail elsewhere herein.

SUMMARY OF THE INVENTION

In an embodiment of the invention, a method is performed for a system including a first storage system, a second storage system, and a first logical storage unit, where the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system. The method includes: recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and, in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system. The replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit. The replicating of the plurality of snapshots may be performed as part of a first process that is executed independently of one or more processes executing the replicating of the write operations. The plurality of snapshots may be listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and replicating the plurality of snapshots may include replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, which may include, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and, if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system. Replicating the first snapshot to the first replica snapshot may include, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and, if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system. The plurality of snapshots may be listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and replicating the plurality of snapshots may include, after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, which may include: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot. The method further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and, if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system. The method further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot, determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system, and, if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data.

In another embodiment of the invention, a system includes a first storage system, a second storage system, a first logical storage unit, where the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system, and executable logic that implements a method. The method includes: recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and, in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system. The replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit. The replicating of the plurality of snapshots may be performed as part of a first process that is executed independently of one or more processes executing the replicating of the write operations. The plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and replicating the plurality of snapshots may include replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, which may include, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and, if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system. Replicating the first snapshot to the first replica snapshot may include, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and, if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system. The plurality of snapshots may be listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and replicating the plurality of snapshots may include, after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, which may include: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot. The method further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and, if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system. The method further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and then updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data.

In another embodiment of the invention, computer-readable media having software stored thereon is provided for a system including a first storage system, a second storage system, and a first logical storage unit, wherein the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system. The computer-readable media has software stored thereon including: executable code that controls, recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and executable code that controls, in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system. The replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit. The plurality of snapshots may be listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and replicating the plurality of snapshots may include replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, which may include, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and, if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system. Replicating the first snapshot to the first replica snapshot may include, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and, if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system. The plurality of snapshots may be listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein replicating the plurality of snapshots may include, after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, which may include: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot. The software further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: executable code that controls determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and executable code that controls, if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system. The software further may include, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: executable code that controls determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and executable code that controls, if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become more apparent from the following detailed description of illustrative embodiments thereof taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram illustrating an example of a data storage network, according to embodiments of the invention;

FIG. 2 is a block diagram illustrating an example of a storage system including multiple physically discrete storage processing nodes, according to embodiments of the invention;

FIG. 3 is a block diagram illustrating an example of tables defining relationships between logical storage units and physical storage devices on a data storage system, according to embodiments of the invention;

FIG. 4 a block diagram illustrating an example of a table used for a thin logical device, according to embodiments of the invention;

FIG. 5 is a block diagram illustrating an example of a data structure for mapping logical storage unit tracks to cache slots, according to embodiments of the invention;

FIGS. 6A-6C are examples of various embodiments of components configured for replication, according to embodiments of the invention;

FIG. 7A is a diagram illustrating an example of a replication data pointer table, according to embodiments of the invention;

FIG. 7B is a diagram illustrating an example of a replication data pointer tree, according to embodiments of the invention;

FIG. 8 is a diagram illustrating an example of a data pool, according to embodiments of the invention;

FIG. 9 is a diagram illustrating an example of a snapshot table, according to embodiments of the invention;

FIG. 10 is a diagram s illustrating an example of a sequence number pointer table, according to embodiments of the invention;

FIG. 11 is a flow diagram illustrating processing performed in connection with initiating a targetless snapshot, according to embodiments of the invention;

FIG. 12 is a flow diagram illustrating processing performed in connection with a write to a logical device after initiating a targetless snapshot, according to embodiments of the invention;

FIG. 13 is a flow diagram illustrating processing performed in connection with a read operation after initiating a targetless snapshot, according to embodiments of the invention;

FIG. 14 is a flow diagram illustrating an example of a method of updating outstanding snapshot information for write operations received while remote replication is inactive, according to embodiments of the invention; and

FIGS. 15A and 15B present a flow diagram illustrating an example of a method of remotely replicating outstanding snapshots, according to embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

On some data storage networks, remote data replication is employed between two or more storage systems, where LSUs from each storage system are logically paired so that the data of an LSU (R1) on one storage system (A1), which may be referred to herein a “primary storage system,” is remotely replicated to another LSU (R2, e.g., a replica LSU) on the other storage system (A2), which may be referred to herein as a secondary storage system. This remote replication may be performed synchronously (synchronous replication) or asynchronously (asynchronous replication), as described in more detail elsewhere herein.

On some storage systems today, local replication (i.e., to another location on a same storage system) of an LSU may be performed using snapshots, where a snapshot defines a point-in-time image of an LSU; i.e., the state of an LSU at the point in time. There are several known techniques for implementing snapshots, including those described in U.S. Pat. No. 7,340,489 to Vishlitzky, et al. titled “Virtual Storage Devices,” issued Mar. 4, 2008, U.S. Pat. No. 9,965,216 to Jaganathan et al., titled “Targetless Snapshots,” issued May 8, 2018 (“Jaganathan”), and U.S. patent application Ser. No. 16/885,702 to Tobin et al., titled “Snapshot Metadata Deduplication,” filed May 28, 2020 (“Tobin”), the entire contents of each of which is hereby incorporated by reference.

It may be desirable to remotely replicate snapshots from one storage system (A1) to another storage system (A2). If remote connectivity between the A1 and A2 is somehow lost, or remote replication otherwise ceases to be operational (i.e., becomes inactive) between R1 and R2, snapshots cannot be replicated. Snapshots of R1 taken when remote replication is inactive between R1 and R2 such that the snapshots have not yet been replicated to R2 may be referred to herein as “outstanding snapshots.” Outstanding snapshots also include snapshots of R1 that are taken before remote replication between R1 and R2 that have not completed (and perhaps not even started) being replicated to R2 when remote replication between R1 and R2 becomes inactive. When remote replication between R1 and R2 is re-established (i.e., becomes active again), it may be desirable to remotely replicate the outstanding snapshots to R2.

In some embodiments of the invention, remote replication of outstanding snapshots may be managed manually, for example, via a host system. For example, when targetless snapshots are employed, a user may manually link an LSU (a target LSU) to an outstanding snapshot of R1 on A1, e.g., as described in Jaganathan or Tobin. The target LSU may be configured for standard replication (e.g., Dell EMC™ SRDF® as described in more detail herein) such that the target LSU is replicated to A2. The user also may configure A2 to create a snapshot of the target LSU once it is fully replicated to A2, thereby producing a snapshot on A2 that is a replica of the outstanding snapshot of R1 to which the target LSU was linked. However, the foregoing approach requires that resources engaged in the remote replication of R1 to R2—where such remote replication also may be in the process of recovering from inactivity—be used for snapshot data transfer instead. Thus, until all snapshots are fully remotely synchronized, which could take a long time depending on the number of outstanding snapshots (perhaps hundreds or even thousands) and data change rate, remote replication could be compromised or unavailable. Such compromise and unavailability likely would not be tolerated by mission critical applications.

What may be desirable is a way to remotely replicate outstanding snapshots that does not have the shortcomings described above.

Described herein are techniques and mechanisms for replicating outstanding snapshots from a first LSU (R1) on a first storage system (A1) to a second replica LSU (R2) on a second storage system (A2) that does not disrupt or impair the remote replication of write operations between R1 and R2; e.g., as part of standard remote replication. A process, P, on A1 executing the replication of the outstanding snapshots from R1 to R2 may be a separate process than the one or more processes on A1 executing standard remote replication including the replication of write operations from R1 to R2. In addition to performing standard replication and replicating outstanding snapshots, e.g., in response to replication connectivity being re-established or replication being otherwise re-activated, A1 also may be copying outstanding write operations that occurred when replication was inactive to A2. In such embodiments, P1 may be run independently from the one or more processes copying the outstanding write operations and/or performing standard remote replication.

In some embodiments, the process P may be given low priority on A1 so as to not impair performance of other operations (including standard remote replication and copying outstanding write operations) on A1. For example, the process P may be run as a background process on A1. The resulting replicated snapshots on R2 may be identical to the snapshots on R1 that they replicate, and the snapshots on R2 may be dependent-write consistent (i.e., write consistent) in that, if a failure occurs at any point during the process of replicating the outstanding snapshots, a write consistent copy of snapshot data will remain on R2, as a result of applying the techniques described herein.

While remote replication is inactive between R1 and R2, any snapshot that had not yet been replicated to R2 at the time the inactivity began or that is taken during the inactivity period shall be marked as outstanding. For example, one or more snapshot data structures may include a field that may be used to indicate that the snapshot is outstanding. Such a field may be configured to be set with a binary value that is initially set to indicate that the snapshot has not been remotely replicated, and is changed to indicate that the remote replication is complete when remote replication of the snapshot is completed. As used herein, the “taking a snapshot” means updating snapshot metadata for an LSU so that a latest snapshot reflects the state of an LSU (e.g., R1) at the current point in time (i.e., the point in time the snapshot is taken). The terminology “taking a snapshot” may be used interchangeably herein with “activating a snapshot.” It should be appreciated that a snapshot may be created prior to being taken or activated in that that basic parameters (e.g., an ID, sequence number) of the snapshot may be defined, for example, as described in Jaganathan, before the snapshot is taken.

Also during the period of time during which remote replication is inactive between R1 and R2, individual logical storage elements (LSEs) of R1 may receive at least one write operation after a snapshot of R1 is taken. LSEs, for example, tracks, sub-portions thereof, or other types of elements or portions thereof, are described in more detail elsewhere herein. When at least one write operation is performed after a snapshot is taken, snapshot metadata for the LSE may be marked to indicate that the LSE is dirty for the snapshot meaning that snapshot data created for the LSE as a result of the at least one write after the snapshot being taken has not been replicated yet to R2. Snapshot data is data that is persisted (e.g., stored) exclusively for use by snapshots to reflect the state of LSUs at the different points in time represented by the snapshots. For example, in some snapshot technologies, such as those described in Jaganathan and Tobin, current LSU data may be shared by a snapshot of the LSU initially; at the time at which the snapshot is taken. That is, after a snapshot is taken, but before any write operations to any LSEs of the LSU following the snapshot being taken, the snapshot metadata points, directly or indirectly, to the same data pointed to by the LSU metadata representing the current state of the LSU. It is only after a first write to an LSE of the LSU following the taking of the snapshot that the snapshot data and the LSU data may diverge, resulting in the moving of the old data for the LSE to a new location (e.g., from a pool of storage reserved for snapshots), and an updating of the snapshot metadata to point, directly or indirectly, to the data at the new location. This old data pointed-to by the snapshot metadata is now snapshot data that is exclusively used for snapshots, as opposed to the new data of the write operation, the location of which is now pointed to by current LSU metadata for the LSE, and also may be shared by future snapshots. Examples of data structures for storing LSU metadata are described in more detail herein.

It should be appreciated that, as used herein, a thing (e.g., metadata) that “references” or “points to” another thing (e.g., data) without a qualifier of “directly” or “indirectly” may either directly or indirectly reference or point to the thing. For example, a metadata value that points to or references data may do so directly by specifying the storage location itself of the data, or may do so indirectly by referencing or pointing to other metadata (e.g., in another data structure) that may specify the storage location itself of the data or reference/point to other metadata (and so on) that ultimately specifies the storage location itself of the data.

After remote replication between R1 and R2 has been restored; i.e., the remote replication is active again, the outstanding snapshots may be replicated, for example, in a chronological order in which they were taken. For a first outstanding snapshot (e.g., the earliest taken snapshot of the outstanding snapshots), each LSE of R1 may be analyzed to determine whether snapshot data for the LSE has not been replicated to R2 (e.g., the snapshot data is marked as dirty), and if so, the snapshot data may be replicated to R2 and snapshot metadata on R1 and R2 updated accordingly. During this analysis for the LSE, before determining whether the snapshot data for LSE is dirty, it may be determined whether the LSE itself is replication dirty—i.e., whether any outstanding write operations for the LSE has not been copied to R2. If the LSE is replication dirty, the replication of the snapshot data of the LSE, or of R1 as a whole, may wait until the LSE data is replication clean—i.e., until the most recent outstanding write of the LSE is copied to R2—before proceeding further with the replication of the first outstanding snapshot.

In some embodiments, if the LSE is replication dirty, rather than wait for the LSE to become clean (i.e., for the most recent outstanding write for the LSE to be copied to R2), the replication process may record that the LSE is replication dirty (e.g., in a data structure), and continue snapshot replication for other LSEs of R1 for the first outstanding snapshot. The snapshot replication process may do the same for any LSEs of R1 for which the LSE is replication dirty. For example, a list of LSEs that are replication dirty may be maintained. After snapshot replication processing for the last LSE of R1 is completed, the snapshot replication processing may return to the LSEs that were previously determined to be replication dirty, for example, by accessing the list of such replication-dirty LSEs, which may be done in consideration that the most recent outstanding write operations of the replication-dirty LSEs may have been copied to R2 during the time that has elapsed since last checked, so that the LSEs are replication clean. For any LSE for which the LSE is still marked replication dirty, the process may move on to the next LSE and attempt to complete snapshot replication processing for the next LSE, and repeat the foregoing process until there are no longer any replication-dirty LSEs and snapshot replication processing for the first outstanding snapshot of R1 is completed.

In some embodiments, the snapshot replication processing of the remaining outstanding snapshots (i.e., after the first outstanding snapshot) for R1 is different than the snapshot replication processing of the first outstanding snapshot. For example, for the first outstanding snapshot, each LSE of R1 may be analyzed to determine whether snapshot data for the LSE has not been replicated to R2. In contrast, for each remaining outstanding snapshot, the replication process may include determining differences between the outstanding snapshot and the last (i.e., the immediately preceding) outstanding snapshot; i.e., which LSEs have had their data modified by one or more write operations between the snapshots. Only the LSEs having modified data may be analyzed to determine whether snapshot data for the LSE has not been replicated to R2, and if so, such snapshot data may be replicated to R2 and the snapshot metadata for R2 updated accordingly, as described in more detail elsewhere herein.

It should be appreciated that embodiments of the invention described herein may be applicable to various types of remote replication, including synchronous and asynchronous replication as well as other types of replication, including any types describe herein.

Illustrative embodiments of the invention will now be described in more detail in relation to the figures.

FIG. 1 illustrates an example of an embodiment of a data storage network 10 (often referred to herein as a “storage network”). The storage network 10 may include any of: host systems (i.e., “hosts”) 14 a-n; network 18; one or more storage systems 20 a-n; other components; or any suitable combination of the foregoing. Storage systems 20 a-n, connected to host systems 14 a-n through network 18, may collectively constitute a distributed storage system 20. All of the host computers 14 a-n and storage systems 20 a-n may be located at the same physical site, or, alternatively, two or more host computers 14 a-n and/or storage systems 20 a-n may be located at different physical locations. Storage network 10 or portions thereof (e.g., one or more storage systems 20 a-n in combination with network 18) may be any of a variety of types of storage networks, such as, for example, a storage area network (SAN), e.g., of a data center. Embodiments of the invention are described herein in reference to storage system 20 a, but it should be appreciated that such embodiments may be implemented using other discrete storage systems (e.g., storage system 20 n), alone or in combination with storage system 20 a.

The N hosts 14 a-n may access the storage system 20 a, for example, in performing input/output (I/O) operations or data requests, through network 18. For example, each of hosts 14 a-n may include one or more host bus adapters (HBAs) (not shown) that each include one or more host ports for connecting to network 18. The network 18 may include any one or more of a variety of communication media, switches and other components known to those skilled in the art, including, for example: a repeater, a multiplexer or even a satellite. Each communication medium may be any of a variety of communication media including, but not limited to: a bus, an optical fiber, a wire and/or other type of data link, known in the art. The network 18 may include at least a portion of the Internet, or a proprietary intranet, and components of the network 18 or components connected thereto may be configured to communicate in accordance with any of a plurality of technologies, including, for example: SCSI, ESCON, Fibre Channel (FC), iSCSI, FCoE, GIGE (Gigabit Ethernet), NVMe over Fabric (NVMeoF); other technologies, or any suitable combinations of the foregoing, each of which may have one or more associated standard specifications. In some embodiments, the network 18 may be, or include, a switching fabric including one or more switches and other components. A network located externally to a storage system that connects host systems to storage system resources of the storage system, may be referred to herein as an “external network.”

Each of the host systems 14 a-n and the storage systems 20 a-n included in the storage network 10 may be connected to the network 18 by any one of a variety of connections as may be provided and supported in accordance with the type of network 18. The processors included in the host computer systems 14 a-n may be any one of a variety of proprietary or commercially available single or multi-processor system, such as an Intel-based processor, or other type of commercially available processor able to support traffic in accordance with each particular embodiment and application. Each of the host computer systems may perform different types of I/O operations in accordance with different tasks and applications executing on the hosts. In the embodiment of FIG. 1, any one of the host computers 14 a-n may issue an I/O request to the storage system 20 a to perform an I/O operation. For example, an application executing on one of the host computers 14 a-n may perform a read or write operation resulting in one or more I/O requests being transmitted to the storage system 20 a.

Each of the storage systems 20 a-n may be manufactured by different vendors and interconnected (not shown). Additionally, the storage systems 20 a-n also may be connected to the host systems through any one or more communication connections 31 that may vary with each particular embodiment and device in accordance with the different protocols used in a particular embodiment. The type of communication connection used may vary with certain system parameters and requirements, such as those related to bandwidth and throughput required in accordance with a rate of I/O requests as may be issued by each of the host computer systems 14 a-n, for example, to the storage systems 20 a-20 n. It should be appreciated that the particulars of the hardware and software included in each of the components that may be included in the storage systems 20 a-n are described herein in more detail, and may vary with each particular embodiment.

Each of the storage systems, such as 20 a, may include a plurality of physical storage devices 24 (e.g., physical non-volatile storage devices) such as, for example, disk devices, solid-state storage devices (SSDs, e.g., flash, storage class memory (SCM), NVMe SSD, NVMe SCM) or even magnetic tape, and may be enclosed within a disk array enclosure (DAE) 27. In some embodiments, two or more of the physical storage devices 24 may be grouped or arranged together, for example, in an arrangement consisting of N rows of physical storage devices 24 a-n. In some embodiments, one or more physical storage devices (e.g., one of the rows 24 a-n of physical storage devices) may be connected to a back-end adapter (“BE”) (e.g., a director configured to serve as a BE) responsible for the backend management of operations to and from a portion of the physical storage devices 24. A BE is sometimes referred to by those in the art as a disk adapter (“DA”) because of the development of such adapters during a period in which disks were the dominant type of physical storage device used in storage systems, even though such so-called DAs may be configured to manage other types of physical storage devices (e.g., SSDs). In the system 20 a, a single BE, such as 23 a, may be responsible for the management of one or more (e.g., a row) of physical storage devices, such as row 24 a. That is, in some configurations, all I/O communications with one or more physical storage devices 24 may be controlled by a specific BE. BEs 23 a-n may employ one or more technologies in communicating with, and transferring data to/from, physical storage devices 24, for example, SAS, SATA or NVMe. For NVMe, to enable communication between each BE and the physical storage devices that it controls, the storage system may include a PCIe switch for each physical storage device controlled by the BE; i.e., connecting the physical storage device to the controlling BE.

It should be appreciated that the physical storage devices are not limited to being arranged in rows. Further, the DAE 27 is not limited to enclosing disks, as the name may suggest, but may be constructed and arranged to enclose a plurality of any type of physical storage device, including any of those described herein, or combinations thereof.

The system 20 a also may include one or more front-end adapters (“FAs”) 21 a-n (e.g., directors configured to serve as FAs), which also are referred to herein as host adapters (“HAs”). Each of these FAs may be used to manage communications and data operations between one or more host systems and global memory (GM) 25 b of memory 26. The FA may be, or include, a Fibre Channel (FC) adapter if FC is a technology being used to communicate between the storage system 20 a and the one or more host systems 14 a-n, or may be another type of adapter based on the one or more technologies being used for I/O communications.

Also shown in the storage system 20 a is a remote adapter (“RA”) 40. The RA may be, or include, hardware that includes a processor used to facilitate communication between storage systems (e.g., 20 a and 20 n), such as between two of the same or different types of storage systems, and/or may be implemented using a director.

Storage system 20 a also may include a management module 22, which may be configured (e.g., dedicated) to performing storage management functions or services such as, for example, storage provisioning, device configuration, tier management, other services, or any combination of other services. The management module may be configured to be accessed by only certain personnel (e.g., storage administrators, support engineers) and may have its own dedicated hardware, firmware, software, CPU resources and OS, and may be loaded with one or more applications, tools, CLIs, APIs and the like to enable management. In some embodiments, the management module, or portions thereof, may be located external to storage system 20 a, for example, as part of one of host systems 14 a-n or another separate system connected to storage system 20 a via network 18.

The FAs, BEs and RA may be collectively referred to herein as directors 37 a-n. Each director 37 a-n may be implemented (e.g., in hardware, firmware, software or a combination thereof) on a circuit board that includes memory resources (e.g., at least a segment of GM portion 25 b) and compute resources, for example, one or more processing cores (e.g., as part of a CPU) and/or a CPU complex for processing I/O operations, and that as described in more detail elsewhere herein. There may be any number of directors 37 a-n, which may be limited based on any of a number of factors, including spatial, computation and storage limitations. In an embodiment disclosed herein, there may be up to sixteen directors coupled to the memory 26. Other embodiments may use a higher or lower maximum number of directors.

System 20 a also may include an internal switching fabric (i.e., internal fabric) 30, which may include one or more switches, that enables internal communications between components of the storage system 20 a, for example, directors 37 a-n (FAs 21 a-n, BEs 23 a-n, RA 40, management module 22) and memory 26, e.g., to perform I/O operations. One or more internal logical communication paths may exist between the directors and the memory 26, for example, over the internal fabric 30. For example, any of the directors 37 a-n may use the internal fabric 30 to communicate with other directors to access any of physical storage devices 24; i.e., without having to use memory 26. In addition, one of the directors 37 a-n may be able to broadcast a message to all of the other directors 37 a-n over the internal fabric 30 at the same time. Each of the components of system 20 a may be configured to communicate over internal fabric 30 in accordance with one or more technologies such as, for example, InfiniBand (TB), Ethernet, Gen-Z, another technology, or any suitable combination of the foregoing.

The GM portion 25 b may be used to facilitate data transfers and other communications between the directors 37 a-n in a storage system. In one embodiment, the directors 37 a-n (e.g., serving as FAs or BEs) may perform data operations using a cache 28 that may be included in the GM 25 b, for example, in communications with other directors, and other components of the system 20 a. The other portion 25 a is that portion of memory that may be used in connection with other designations that may vary in accordance with each embodiment. Global memory 25 b and cache 28 are described in more detail elsewhere herein. It should be appreciated that, although memory 26 is illustrated in FIG. 1 as being a single, discrete component of storage system 20 a, the invention is not so limited. In some embodiments, memory 26, or the GM 25 b or other memory 25 a thereof, may be distributed among a plurality of physically discrete processing nodes (e.g., circuit boards) as described in more detail elsewhere herein.

In at least one embodiment, write data received at the storage system from a host or other client may be initially written to cache 28 and marked as write pending. For example, cache 28 may be partitioned into one or more portions called cache slots (which also may be referred to in the field of data storage as cache lines, cache blocks or another name), which may be a of a predefined uniform size, for example, 128 Kbytes. Write data of a write operation received at the storage system may be initially written (i.e., staged) in one or more of these cache slots and marked as write pending. Once written to cache 28, the host (e.g., one of 14 a-n) may be notified that the write operation has completed. At a later time, the write data may be de-staged from cache 28 to one or more physical storage devices 24 a-n, such as by a BE.

The memory 26 may include persistent memory for which for which data stored thereon persists after the process or program that created the data terminates. For example, at least portions of the memory 26 may be implemented using DIMM (or another type of fast RAM memory) that is battery-backed by a NAND-type memory (e.g., flash). In some embodiments, the data in such persistent memory may persist (for at least some period of time) after the storage system fails. The memory 26 (or at least a portion thereof—e.g., the cache 28 or a portion thereof) may be configured such that each data written to the memory 28 is mirrored to provide a form of write protection. For example, each memory location within each such mirrored portion of the memory 26 may have a corresponding memory location on the storage system 20 a to which a redundant copy of the data is stored, and which can be used in place of the mirrored memory location in the event the mirrored memory location fails. The redundant memory location should be located outside of at least the most local fault zone of the mirrored memory location. In some embodiments described in more detail herein, the memory 26 may be distributed among multiple physically discrete processing nodes (e.g., circuit boards), in which case mirroring may be configured such that a mirrored memory location and its corresponding redundant memory location are located on different physically discrete processing nodes.

Storage system 20 a may include a back-up power supply 41 (e.g., a battery) that can provide power to the storage system for a limited amount of time to after primary (AC) power fails. This limited time may allow certain tasks to be performed during a window of time beginning when the primary power fails until the earliest of: the primary power is restored; and the end of the limited lifetime (sometimes on the order of second or tens of seconds) of the back-up power supply. For example, the storage system 20 a (e.g., the memory 26 and/or memory management module 32) may be configured to automatically copy the contents of the memory 26 during this window of time to one or more predetermined physical storage devices, to be restored to the memory 26 after the power has been restored, e.g., as part of the storage system recovering process. Such automatic copying for restoration during recovering may referred to herein as “vaulting.” Vaulting may provide a form of write protection for data written to the memory 26, for example, for dirty data in the cache 28; i.e., data written to the storage system, which has been staged in the cache 28 but not yet de-staged to a physical storage device. More broadly, vaulting may be performed for any data written to the memory 26.

The storage system 20 a may include a memory management module 32 configured to manage one or more aspects of the memory 26, and the memory management module 32 may include a cache management module 34 for managing one or more aspects of the cache 28.

It should be noted that, although examples of techniques herein may be made with respect to a physical storage system and its physical components (e.g., physical hardware for each RA, BE, FA and the like), techniques herein may be performed in a physical storage system including one or more emulated or virtualized components (e.g., emulated or virtualized ports, emulated or virtualized BEs or FAs), and also a virtualized or emulated storage system including virtualized or emulated components. For example, in embodiments in which NVMe technology is used to communicate with, and transfer data between, a host system and one or more FAs, one or more of the FAs may be implemented using NVMe technology as an emulation of an FC adapter.

Any of storage systems 20 a-n, or one or more components thereof, described in relation to FIGS. 1-2 may be implemented using one or more Symmetrix™, VMAX™, VMAX3™ or PowerMax™ systems made available from Dell EMC.

Host systems 14 a-n may provide data and control (e.g., management and access control) information to storage systems 20 a-n over a plurality of I/O paths defined between the host systems and storage systems, for example, including host system components, storage system components, and network components (e.g., of network 18), and the storage systems also may provide data to the host systems across the I/O paths. In the embodiment of FIG. 1, the host systems may not address the physical storage devices (e.g., disk drives or flash drives) 24 of the storage systems directly, but rather access to data may be provided to one or more host systems from what the host systems view as a plurality of LSUs including, for example, logical blocks, logical devices (also referred to as logical volumes, LUNs, logical storage units and/or logical disks), thin devices, groups of logical devices (e.g., storage groups), NVMe namespaces, and other types of LSUs. For example, a PowerMax storage system may be configured to organize available storage resources (e.g., physical storage devices) into many LUNs, each with its own addressable space defined in logical blocks addresses (LBAs). The LSUs may or may not correspond to the actual physical storage devices. For example, one or more LSUs may map to a single physical storage device; that is, the logical address space of the one or more LSU may map to physical space on a single physical storage device. Data in a single storage system may be accessed by multiple hosts allowing the hosts to share the data residing therein. The FAs may be used in connection with communications between a storage system and a host system. The RAs may be used in facilitating communications between two storage systems. The BEs may be used in connection with facilitating communications to the associated physical storage device(s) based on LSU(s) mapped thereto.

FIG. 2 is a block diagram illustrating an example of at least a portion 211 of a storage system (e.g., 20 a) including multiple, physically discrete storage processing nodes (e.g., circuit boards) 212 a-212 n, which may be referred to herein as “processing nodes.” Storage system 211 may include a plurality of processing nodes 212 a-212 n and a fabric 230 (e.g., internal fabric 30) over which the processing nodes 212 a-n may communicate. Each of the processing nodes 212 a-212 n may include components thereon as illustrated. The switching fabric 230 may include, for example, one or more switches and connections between the switch(es) and processing nodes 212 a-212 n. In at least one embodiment, the fabric 230 may be an IB fabric. In some embodiments, multiple processing 212 a-n nodes may be implemented on a single physically discrete component; e.g., two processing nodes 212 a-n may be implemented on single engine of PowerMax storage system.

In the following paragraphs, further details are described with reference to processing node 212 a but each of the N processing nodes in a system may be similarly configured. For example, processing node 212 a may include any of: one or more directors 216 a (e.g., directors 37 a-n); memory portion 214 a; one or more processing cores 217 a including compute resources, for example, as part of a CPUs and/or a CPU complex for processing I/O operations; and a fabric interface module (FIM) 215 a for interfacing the processing node 212 a to an internal fabric 230. Each director 216 a may be configured to operate, such as by executing code, as any one or more of an FA, BE, RA, and the like. In some embodiments, each of the directors, or a portion thereof, are implemented in software stored in a memory portion 214 a (e.g., in a dedicated local memory 222 a) that is executed by one or more of the processing cores 217 a. Such software implementation of directors may be considered emulations of types of physical directors (i.e., directors implemented (at least primarily) in hardware).

Each FIM 215 a-n may include one or more host channel adapters (HCAs) that physically couple, and are configured to enable communication between, its respective processing node 212 a-n, and the internal fabric 230. In some embodiments, the internal fabric 230 may include multiple (e.g., 2) switches, and each HCA 215 a-n may have multiple (e.g., 2) ports, each one connected directly to one of the switches.

Each of the processing nodes 212 a-n may, respectively, also include memory portions 214 a-n. The memory portion of each processing node may be characterized as locally accessible with respect to that particular processing node, and more specifically with respect to other components on the same processing node. For example, processing node 212 a includes memory portion 214 a which is memory that is local to that particular processing node 212 a. Data stored in memory portion 214 a may be directly accessed by any of the processing cores 217 a (e.g., executing instructions on behalf of one of the directors 216 a) of the processing node 212 a. For example, memory portion 214 a may be a fast memory (e.g., DIMM (dual inline memory module) DRAM (dynamic random access memory)) that is locally accessible by a director 216 a, where data from one location in 214 a may be copied to another location in 214 a directly using DMA operations (e.g., local memory copy operations) issued by director 216 a. Thus, the director 216 a may directly access data of 214 a locally without communicating over the fabric 230.

The memory portions 214 a-214 n of processing nodes 212 a-n may be further partitioned into different portions or segments for different uses. For example, each of the memory portions 214 a-214 n may respectively include GM segments 220 a-n configured for collective use as segments of a distributed GM, for example, GM 225 (e.g., GM 25 b). Thus, data stored in any GM segment 220 a-n may be accessed by any director 216 a-n on any processing node 212 a-n. Additionally, each of the memory portions 214 a-n may respectively include dedicated local memories 222 a-n. Each of the dedicated local memories 222 a-n are respectively configured for use locally by the one or more directors 216 a-n, and possibly other components, residing on the same single processing node. In at least one embodiment where there is a single director denoted by 216 a (and generally by each of 216 a-n), data stored in the dedicated local memory 222 a may be accessed by the respective single director 216 a located on the same processing node 212 a. However, the remaining directors located on other ones of the N processing nodes may not access data stored in the dedicated local memory 222 a.

To further illustrate, GM segment 220 a may include information such as user data stored in the cache portion 220 a, metadata, and the like, that is accessed (e.g., for read and/or write) generally by any director of any of the processing nodes 212 a-n. Thus, for example, any director 216 a-n of any of the processing nodes 212 a-n may communicate over the fabric 230 to access data in GM segment 220 a. In a similar manner, any director 216 a-n of any of the processing nodes 212 a-n may generally communicate over fabric 230 to access any GM segment 220 a-n of the distributed GM. A1 though a particular GM segment, such as 220 a, may be locally accessible to directors on one particular processing node, such as 212 a, any director of any of the processing nodes 212 a-n may generally access the GM segment 220 a. Additionally, the director 216 a also may use the fabric 230 for data transfers to and/or from GM segment 220 a even though 220 a is locally accessible to director 216 a (without having to use the fabric 230).

Also, to further illustrate, dedicated local memory 222 a may be a segment of the memory portion 214 a on processing node 212 a configured for local use solely by components on the single/same processing node 212 a. For example, dedicated local memory 222 a may include data described in following paragraphs which is used and accessed only by directors 216 a included on the same processing node 212 a as the dedicated local memory 222 a. In at least one embodiment in accordance with techniques herein and as described elsewhere herein, each of the dedicated local memories 222 a-n may include a local page table or page directory used, respectively, by only director(s) 216 a-n local to each of the processing nodes 212 a-n.

In such an embodiment as in FIG. 2, the GM segments 220 a-n may be logically concatenated or viewed in the aggregate as forming one contiguous GM logical address space of a distributed GM. In at least one embodiment, the distributed GM formed by GM segments 220 a-n may include the cache portion 254 a, various metadata and/or structures, and other information, as described in more detail elsewhere herein. Consistent with discussion herein, the cache portion 254 a, having cache slots allocated from GM segments 220 a-n, may be used to store I/O data (e.g., for servicing read and write operations).

Each cache portion 254 a-n may be a portion of a shared cache 228 (e.g., cache 28) distributed across the processing nodes 212 a-n, where the shared cache 228 may be considered a part of the GM 225. The cache portion 254 a-n may include a plurality of cache slots 256 a-n, each cache slot including one or more (e.g., 16) sections 258 a-n. Each cache slot 256 a-n may be of a uniform size (e.g., 128 KB) and each section may be of a uniform size (e.g., 8 KB). It should be appreciated that cache slot sizes and section sizes other than 128 KB and 8 KB, and a quantity of sections other than 16, may be used.

In an embodiment, the storage system as described may be characterized as having one or more logical mapping layers in which an LSU of the storage system is exposed to the host whereby the LSU is mapped by such mapping layers of the storage system to one or more physical storage devices. Additionally, the host also may have one or more additional mapping layers so that, for example, a host-side LSU may be mapped to one or more storage system LSUs as presented to the host.

Any of a variety of data structures may be used to process I/O on storage system 20 a, including data structures to manage the mapping of LSUs and locations thereon to physical storage devices and locations thereon. Such data structures may be stored in any of memory 26, including GM 25 b and memory 25 a, GM segment 220 a-n and/or dedicated local memories 22 a-n. Thus, storage system 20 a, and storage system 620 a described in more detail elsewhere herein, may include memory elements (e.g., cache) that hold data stored on physical storage devices or that is currently held (“staged”) and will be stored (“de-staged”) to physical storage devices, and memory elements that store metadata (e.g., any of the metadata described herein) associated with such data. Illustrative examples of data structures for holding such metadata will now be described.

FIG. 3 is a block diagram illustrating an example of tables 60 defining relationships between LSUs and physical storage devices on a data storage system, according to embodiments of the invention. A first table 62 corresponds to the LSUs (e.g., logical deices) used by a storage system (e.g., storage system 20 a) or by an element of a storage system, such as an FA and/or a BE, and may be referred to herein as a “master LSU table.” The master LSU table 62 may include a plurality of LSU entries 66-68, each entry representing an LSU used by the storage system. The entries in the master LSU table 62 may include descriptions for any type of LSU described herein.

Each of the entries 66-68 of the master LSU table 62 may correspond to, and include a reference to, another table corresponding to the LSU represented by the respective entry. For example, the entry 67 may reference a table 72, referred to herein as an “LSU table,” corresponding to the LSU represented by the entry 67. The LSU table 72 may include a header that contains information pertinent to the LSU as a whole. The LSU table 72 also may include entries 76-78 for separate contiguous logical data portions of the represented LSU; each such logical data portion corresponding to, and including a reference to, one or more contiguous physical locations (e.g., logical block address ranges) of a physical storage device (e.g., a cylinder and/or a group of tracks). In an embodiment disclosed herein, an LSU may contain any number of logical data portions depending upon how the LSU is initialized. However, in other embodiments, an LSU may contain a fixed number of logical data portions.

Each of the logical data portion entries 76-78 may correspond to a track table. For example, the entry 77 may correspond to a track table (or “LSU track table”) 82, which includes a header 84. The LSU track table 82 also includes entries 86-88, each entry representing an LSU track of the entry 77. As used herein, a “track” or “LSU track” represents a contiguous segment of physical storage space on a physical storage device. In an embodiment disclosed herein, there are fifteen tracks for each contiguous logical data portion. However, for other embodiments, it may be possible to have different numbers of tracks for each of the logical data portions or even a variable number of tracks for each logical data portion. The information in each of the LSU track entries 86-88 may include a pointer (either direct or indirect—e.g., through another data structure) to a physical address of a physical storage device, for example, any of physical storage devices 24 of the storage system 20 a (or a remote storage system if the system is so configured).

In addition to physical storage device addresses, or as an alternative thereto, each of the LSU track entries 86-88 may include a pointer (either direct or indirect—e.g., through another data structure) to one or more cache slots of a cache in the GM if the data of the logical track is currently in cache. For example, an LSU track entry 86-88 may point to one or more entries of cache slot table 300, described in more detail elsewhere herein. Thus, the LSU track table 82 may be used to map logical addresses of an LSU corresponding to the tables 62, 72, 82 to physical addresses within physical storage devices of a storage system and/or to cache slots within a cache. In some embodiments, each entry 86-88 may specify a version of the data stored on the track. A sub-element of an LSU, for example, a logical storage portion or track, may be referred to herein as a logical storage element (LSE).

FIG. 4 is a diagram illustrating an example of a table 72′ used for a thin logical device (i.e., a thin LSU), which may include null pointers as well as entries similar to entries for the LSU table 72, discussed above, that point to a plurality of LSU track tables 82 a-82 e. Table 72′ may be referred to herein as a “thin device table.” A thin logical device may be allocated by the system to show a particular storage capacity while having a smaller amount of physical storage that is actually allocated. When a thin logical device is initialized, all (or at least most) of the entries in the thin device table 72′ may be set to null. Physical data may be allocated for particular sections as data is written to the particular logical data portion. If no data is written to a logical data portion, the corresponding entry in the thin device table 72′ for the logical data portion maintains the null pointer that was written at initialization.

FIG. 5 is a block diagram illustrating an example of a data structure 300 for mapping LSU tracks (e.g., thin device tracks) to cache slots of a cache. Data structure 300 may be referred to herein as a “cache slot table.” The cache slot table 300 may include a plurality of entries (i.e., rows) 302, each row representing an LSU track (e.g., any of LSU tracks 86-88 in track table 82) identified by an LSU ID in column 304 and an LSU track ID (e.g., number) identified in column 306. For each entry of the cache slot table 300, a column 312 may specify (e.g., using a cache slot ID and/or memory address) a cache location in a cache corresponding to the logical storage device track specified by columns 304 and 306. A combination of an LSU identifier and LSU track identifier may be used to determine from columns 304 and 306 whether the data of the identified LSU track currently resides in any cache slot identified in column 312. Through use of information from any of tables 62, 72, 72′ and 82 described in more detail elsewhere herein, the one or more LSU tracks of an LSU specified in an I/O operation can be mapped to one or more cache slots. Further, using the same data structures, the one or more physical address ranges corresponding to the one or more LSU tracks of the LSU may be mapped to one or more cache slots.

Each of the entries 302 of the cache slot table also may specify: cache lock information in a column 314, replication information in a column 316, and other cache information in a column 318. The cache lock information may indicate whether or not the cache slot represented by the entry is locked, and if locked, the process ID of the entity that owns the lock. The entity may be, for example: an FA executing a write operation from a host; an RA replicating a write operation from the cache slot to R2, or replicating a write operation from R2 into the cache slot; or a BE de-staging data in the cache to a physical storage device or reading data from a PSD into the cache slot. The replication information may specify information relative to replication, for example, the replication cycle number currently associated with the cache slot, the replication (e.g., RDF) group associated with the cache slot (i.e., associated with the R1 track currently mapped to the cache slot, a type of cache slot (e.g., normal or duplicate), and other information. A normal cache slot type may indicate that a cache slot is handled per normal processing, i.e., when there is not a cache lock conflict resolution involved, for example, as described herein. A duplicate cache slot type may indicate that a cache slot is a duplicate of a cache slot used to resolve a cache slot lock conflict, which is not handled in the standard manner, but rather, is handled differently to resolve the cache slot lock, for example, as described herein.

The other cache slot information in the column 318 may include information about the status of writes to one or more portions (e.g., sectors) of the R1 track corresponding to the cache slot, e.g., whether the write is pending or complete. Completing the write may include writing it to a PSD on A1 (e.g., de-staging it from cache) and receiving acknowledgement from A2 (and perhaps other remote storage systems to which the LSU in question is being replicated) that the replicated data is committed on A2 (and other remote storage systems if any).

It should be appreciated that the cache slot table 300 may be used for purposes independent of any LSU tracks mapped thereto. That is, a cache slot ID or memory address in cache pointer column 312 may be used as a key to access, and modify as necessary, cache metadata about a cache slot, including any of the information in columns 314, 316 and/or 318.

The tables 62, 72, 72′, 82 and 300 may be stored in the GM 26 of the storage system 20 a during operation thereof and may otherwise be stored in non-volatile memory (i.e., with the corresponding physical storage device). In addition, tables corresponding to LSUs accessed by a particular host may be stored in local memory of the corresponding one of the FAs 21 a-n. In addition, RA 40 and/or the BEs 23 a-n also may use and locally store portions of the tables 62, 72, 72′, 82 and 300. Other data structures may be stored in any of GM 25 b, memory 25 a, GM segment 220 a-n and/or dedicated local memories 22 a-n.

Any of the information contained in any of the data structures 62, 72, 72′, 82 and 300, for example, the information included in the LSU track table 82 and the cache slot table 300, may be combined in a single data structure, which may be referred to herein as an LSU track metadata table. In some embodiments, a cache slot table 300 may be maintained separately from an LSU track metadata table. In such embodiments, the entries 302 of the cache slot table 300 may be indexed/keyed by a cache slot ID and/or memory address in the column 312, may identify the LSU track currently mapped to the slot (if any) in columns 304 and 306, may include cache lock info in the column 314, and may include other cache info. In such embodiments, the LSU track table may include: information about the LSU track described in relation to the LSU track table 82; replication information described in relation to the column 316; the cache slot (of any) currently mapped to the LSU track; and any other information described in relation to the cache slot table 300.

In some embodiments of the invention, data replication may be employed between two or more storage systems on a storage network, which may before referred to herein as “remote data replication” to distinguish it from “local data replication,” which may be used herein to refer to data replication performed within a single storage system. Referring back to FIG. 1, the RA (remote adapter) 40 may be configured to facilitate communication between data storage systems, such as between two of the same or different types of data storage systems. In one embodiment described in more detail in following paragraphs and figures, the RAs of the different data storage systems may communicate over a Gigabit Ethernet or Fibre Channel transmission channel supporting messaging traffic between data storage systems. The RA (e.g., RA 40) may include hardware including a processor used to facilitate communication between data storage systems, such as between two data storage systems. The RA may be used with the Dell EMC™ Symmetrix® Remote Data Facility (SRDF®) products. Dell EMC™ SRDF® is a family of products that facilitates the data replication from one data storage array to another through a Storage Area Network (SAN) or and IP network. Dell EMC™ SRDF® logically pairs a device or a group of devices from each array and replicates data from one to the other synchronously or asynchronously. Generally, the Dell EMC™ SRDF® products are one example of commercially available products that may be used to provide functionality of a remote data facility (RDF) for use in an embodiment in connection with techniques herein.

Referring to FIG. 6A, shown is an example of an embodiment of a system 2101 that may be used in connection with the techniques described herein. It should be noted that the embodiment illustrated in FIG. 6A presents a simplified view of some of the components illustrated in FIG. 1, for example, including only some detail of the data storage system 20 a for the sake of illustration.

Included in the system 2101 are data storage systems 2102 and 2104 and hosts 2110 a, 2110 b and 1210 c. The data storage systems 2102, 2104 may be remotely connected and communicate over network 2122, such as the Internet or other private network, and facilitate communications with the components connected thereto. Hosts 2110 a, 2110 b and 2110 c may perform operations to data storage system 2102 over connection 2108 a. The hosts 2110 a, 2110 b and 2110 c may be connected to the data storage system 2102 through connection 2108 a which may be, for example, network or other type of communication connection. Although not illustrated, the hosts 2110 a-2110 c also may be directly connected to a network such as the Internet.

The data storage systems 2102 and 2104 may include one or more LSUs (e.g., logical storage devices). In this example, data storage system 2102 includes R1 2124 and data storage system 104 includes R2 2126. LSUs R1 and R2 may be referred to herein simply as “R1” and “R2.” Both of the data storage systems may include one or more other logical and/or physical devices. Data storage system 2102 may be characterized as local with respect to hosts 2110 a, 2110 b and 2110 c. Data storage system 104 may be characterized as remote with respect to hosts 2110 a, 2110 b and 2110 c. Each of R1 and R2 may be configured as LUNs.

The host 2110 a may issue a command, such as to write data to R1 of data storage system 2102. In some instances, it may be desirable to copy data from the R1 to another second LSU, such as R2, provided in a different location so that if a disaster occurs that renders R1 inoperable, the host (or another host) may resume operation using the data of R2. Such a capability is provided, for example, by the Dell EMC™ SRDF® products. Communication between LSUs on different data storage systems using Dell EMC™ SRDF® is described, for example, in U.S. Pat. Nos. 5,742,792, 5,544,347, and 7,054,883, all of which are incorporated by reference herein. With Dell EMC™ SRDF®, a user may denote a first LSU, such as R1, as a master LSU and a second LSU, such as R2, as a slave LSU. Other incarnations of Dell EMC™ SRDF® may provide a peer to peer relationship between the local and remote LSUs. In this example, the host 2110 a interacts directly with the R1 of data storage system 2102, but any data changes made are automatically provided to the R2 LSU of data storage system 2104 using Dell EMC™ SRDF®. In operation, the host 2110 a may read and write data using the R1 volume in 2102, and Dell EMC™ SRDF® may handle the automatic copying and updating of data from R1 to R2 in data storage system 2104.

As illustrated in connection with other figures herein, data storage system 2102 may have one or more RAs included therein to facilitate remote connections to the data storage system 2104. Communications between storage system 2102 and 2104 may be made over connections 2108 b,2108 c to network 2122. Data storage system 2104 may include one or more RAs for use in receiving the communications from the data storage system 2102. The data storage systems may communicate, for example, over Gigabit Ethernet connections supporting TCP/IP traffic. The Dell EMC™ SRDF® replication functionality may be facilitated with the RAs provided at each of the data storage systems 2102 and 2104. Performing remote data communications using SRDF® over a TCP/IP network is described in more detail in U.S. Pat. No. 6,968,369, Nov. 22, 2005, Veprinsky, et al., “Remote Data Facility Over an IP Network,” which is incorporated by reference herein. In connection with Dell EMC™ SRDF®, a single RDF link, connection or path may be between an RA of the system 2102 and an RA of the system 2104. As described in more detail below, techniques are described for use in transmitting data over an RDF link, such as I/O traffic including write data in connection with performing remote data replication over the RDF link between the systems 2102 and 2104.

An embodiment also may include the concept of a remote data facility (RDF) group in which one or more LSUs (e.g., LUNs) on a data storage system are associated with a particular group under the control of a single RA which services the LSUs included therein. Rather than have a single R1 LSU and a single R2 LSU, a grouping may be defined so that a source group of LSUs, such as on data storage system 2102, have corresponding target LSUs of a target group, such as LSUs on data storage system 2104. Devices in a source group may be mirrored in corresponding LSUs of a target group using Dell EMC™ SRDF® functionality.

Techniques herein may be used with Dell EMC™ SRDF®, or more generally any RDF, operating in one or more different supported modes. For example, such modes may include Dell EMC™ SRDF® operating in synchronous mode, asynchronous mode, or adaptive copy mode. For example, in connection with Dell EMC™ SRDF®, the host may issue a write to an R1 LSU in a first data storage system and the data change is propagated to the R2 LSU in a second data storage system. As discussed in U.S. Pat. No. 5,544,347, Dell EMC™ SRDF® can be operated in either a synchronous mode or an asynchronous mode. When operating in the synchronous mode, the host does not consider a write I/O operation to be complete until the write I/O has been completed on both the first and second data storage systems. Thus, in synchronous mode, the first or source storage system will not provide an indication to the host that the write operation is committed or complete until the first storage system receives an acknowledgement from the second data storage system regarding completion or commitment of the write by the second data storage system. In contrast, in connection with the asynchronous mode, the host receives an acknowledgement from the first data storage system as soon as the information is committed to the first data storage system without waiting for an acknowledgement from the second data storage system.

Depending on the physical distance between the data storage systems 2102, 2104, it may be desirable to operate in a mode such as asynchronous to avoid host timeouts while the host awaits acknowledgement regarding completion of a host I/O.

Described in following paragraphs are techniques that may be used in connection with performing data replication in a synchronous manner such as Dell EMC™ SRDF® operating in an synchronous mode (Dell EMC™ SRDF®/S). With synchronous mode data replication, a host 2110 a may issue a write to the R1 LSU 2124. The primary or R1 data storage system 2102 may store the write data in its cache at a cache location and mark the cache location as including write pending (WP) data as mentioned elsewhere herein. The remote data replication facility operating in synchronous mode, such as Dell EMC™ SRDF®/S, may propagate the write data across an established RDF link (more generally referred to as a the remote replication link or link) such as over 2108 b, 2122, and 2108 c, to the secondary or R2 data storage system 2104 where the write data may be stored in the cache of the system 2104 at a cache location that is marked as WP. Once the write data is stored in the cache of the system 2104 as described, the R2 data storage system 2104 may return an acknowledgement to the R1 data storage system 2102 that it has received the write data. Responsive to receiving this acknowledgement from the R2 data storage system 2104, the R1 data storage system 2102 may return an acknowledgement to the host 2110 a that the write has been received and completed. Thus, generally, R1 LSU 2124 and R2 LSU 2126 may be logical devices, such as LUNs, configured as mirrors of one another. R1 and R2 LSUs may be, for example, fully provisioned LUNs, such as thick (i.e., not thin or virtually provisioned) LUNs, or may be LUNs that are thin or virtually provisioned logical devices.

When operating in asynchronous mode when processing a received write I/O operation from a host as noted above, the primary or R1 data storage system 2102 may store the write data in its cache at a cache location and mark the cache location as including write pending (WP) data as mentioned elsewhere herein. The write data may be propagated across an established RDF link (more generally referred to as a the remote replication link or link) such as over 2108 b, 2122, and 2108 c, to the secondary or R2 data storage system 2104 where the write data may be stored in the cache of the system 2104 at a cache location that is marked as WP. Once the write data is stored in the cache of the system 2104 as described, the R2 data storage system 2104 may return an acknowledgement to the R1 data storage system 2102 that it has received the write data. With asynchronous mode, once the write data is stored in the cache of the local or R1 system 2102 and marked as WP, an acknowledgement regarding completion of the host write may be sent to the host 2110 a by the system 2102. Thus, in asynchronous mode the system 2102 is not required to wait to receive the acknowledgement from the R2 data storage system 2104 prior to sending the acknowledgement to the host regarding completion of the write operation.

With reference to FIG. 6B, shown is a further simplified illustration of components that may be used in an embodiment in accordance with techniques herein. The example 2400 is simplified illustration of components as described in connection with FIG. 2. Element 2402 generally represents the replication link used in connection with sending write data from the primary R1 data storage system 2102 to the secondary R2 data storage system 2104. Link 2402, more generally, may also be used in connection with other information and communications exchanged between 2101 and 2104 for replication. As mentioned above, when operating in synchronous replication mode, host 2110 a issues a write, or more generally, all I/Os including reads and writes, over a path to only the primary R1 data storage system 2102. The host 2110 a does not issue I/Os directly to the R2 data storage system 2104. The configuration of FIG. 6B may also be referred to herein as an active-passive configuration such as may be used with synchronous replication and other supported replication modes where the host 2110 a has an active connection or path 2108 a over which all I/Os are issued to only the R1 data storage system. The host 2110 a may have a passive connection or path 2404 to the R2 data storage system 2104. In the configuration of 2400, the R1 LSU 2124 and R2 LSU 2126 may be configured and identified as the same LSU, such as LSU A, to the host 2110 a. Thus, the host 2110 a may view 2108 a and 2404 as two paths to the same LSU A where path 2108 a is active (over which I/Os may be issued to LSU A) and where path 2404 is passive (over which no I/Os to the LSU A may be issued). Should the connection 2108 a and/or the R1 data storage system 2102 experience a failure or disaster whereby access to R1 2124 configured as LSU A is unavailable, processing may be performed on the host 2110 a to modify the state of path 2404 to active and commence issuing I/Os to the R2 LSU configured as LSU A. In this manner, the R2 LSU 2126 configured as LSU A may be used as a backup accessible to the host 2110 a for servicing I/Os upon failure of the R1 LSU 2124 configured as LSU A.

It should be noted although only a single RDF link 2402 is illustrated, more generally any number of RDF links may be used in connection with replicating data from systems 2102 to system 2104 in connection with techniques herein.

Referring to FIG. 6C, shown is another example configuration of components that may be used in an embodiment in accordance with techniques herein. The example 2500 illustrates an active-active configuration as may be used in connection with synchronous replication in at least one embodiment in accordance with techniques herein. In an active-active configuration with synchronous replication, the host 2110 a may have a first active path 2108 a to the R1 data storage system and R1 LSU 2124 configured as LSU A. Additionally, the host 2110 a may have a second active path 2504 to the R2 data storage system and R2 LSU 2126 configured as LSU A. From the view of the host 2110 a, paths 2108 a and 2504 appear as 2 paths to the same LSU A as described in connection with FIG. 6B with the difference that the host in the example 2500 configuration may issue I/Os, both reads and/or writes, over both of paths 2108 a and 2504. The host 2110 a may send a first write over path 2108 a which is received by the R1 system 2102 and written to cache of the R1 system 2102 where, at a later point in time, the first write is de-staged from the cache of the R1 system 2102 to physical storage provisioned for the R1 LSU 2124 configured as LSU A. The R1 system 2102 also sends the first write to the R2 system 2104 over link 2402 where the first write is written to cache of the R2 system 2104, where, at a later point in time, the first write is de-staged from the cache of the R2 system 2104 to physical storage provisioned for the R2 LSU 2126 configured as LSU A. Once the first write is written to the cache of the R2 system 2104, the R2 system 2104 sends an acknowledgement over link 2402 to the R1 system 2102 that it has completed the first write. The R1 system 2102 receives the acknowledgement from the R2 system 2104 and then returns an acknowledgement to host 2110 a over path 2108 a that the first write has completed.

The host 2110 a may also send a second write over path 2504 which is received by the R2 system 2104 and written to cache of the R2 system 2104 where, at a later point in time, the second write is de-staged from the cache of the R2 system 2104 to physical storage provisioned for the R2 LSU 2126 configured as LSU A. The R2 system 2104 also sends the second write to the R1 system 2102 over a second link 2502 where the second write is written to cache of the R1 system 2102, and where, at a later point in time, the second write is de-staged from the cache of the R1 system 2102 to physical storage provisioned for the R1 LSU 2124 configured as LSU A. Once the second write is written to the cache of the R1 system 2102, the R1 system 2102 sends an acknowledgement over link 2502 to the R2 system 2104 that it has completed the second write. Once the R2 system 2104 receives the acknowledgement from the R1 system (regarding completion of the second write), the R2 system 2104 then returns an acknowledgement to host 2110 a over path 2504 that the second write has completed.

Thus, in the example 2500, the illustrated active-active configuration includes a first RDF R1-R2 LSU pairing configured for synchronous replication (from 2102 to 2104) where the R1 LSU is 2124 and the R2 LSU is 2126 whereby writes to LSU A sent over 2108 a to system 2102 are stored on the R1 LSU 2124 and also transmitted to system 2104 over 2402. The write sent over 2402 to system 2104 is stored on the R2 LSU 2126. Such replication is performed synchronously in that the acknowledgement to the host write sent over 2108 a is not acknowledged as successfully completed unless and until the write data has been stored in caches of systems 2102 and 2104.

In a similar manner, the illustrated active-active configuration of the example 2500 includes a second RDF R1-R2 LSU pairing configured for synchronous replication (from 2104 to 2102) where the R1 LSU is 2126 and the R2 LSU is 2124 whereby writes to LSU A sent over 2504 to system 2104 are stored on the LSU 2126 (now acting as the R1 LSU of the second RDF LSU pairing) and also transmitted to system 2102 over connection 2502. The write sent over 2502 is stored on the R2 LSU 2124. Such replication is performed synchronously in that the acknowledgement to the host write sent over 2504 is not acknowledged as successfully completed unless and until the write data has been stored in caches of systems 2102 and 2104.

Effectively, using the second RDF LSU pairing in the active-active configuration with synchronous replication as in FIG. 6C has the R2 system 2104 act as another primary data storage system which facilitates propagation of writes received at the data storage system 2104 to the data storage system 2102. It should be noted that although FIG. 6C illustrates for simplicity a single host accessing both the R1 LSU 2124 and R2 LSU 2126, any number of hosts may access one or both of the R1 LSU 2124 and the R2 LSU 2126.

Although only a single RDF link 2402 is illustrated in connection with replicating data from systems 2102 to system 2104 in connection with techniques herein, more generally any number of RDF links may be used. Although only a single RDF link 2502 is illustrated in connection with replicating data from systems 2104 to system 2102, more generally any number of RDF links may be used. Furthermore, although 2 RDF links 2402 and 2502 are illustrated, in at least one embodiment, a single RDF link may be used in connection with sending data from system 2102 to 2104, and also from 2104 to 2102.

In at least one embodiment in accordance with techniques herein, the FC protocol may be used in connection with communications (e.g., over the SAN including the RDF links) between the data storage system 2102 and 2104.

Asynchronous remote replication (ARR) may include a plurality of replication cycles for an LSU (R1) in a source (e.g.,) storage system (A1), each cycle corresponding to a period of time and specifying any R1 tracks for which data was updated (e.g., by a write operation) during the period of time represented by the cycle. Each cycle may transition through four phases—two phases on A1 and two phases on a target (e.g., secondary) storage system A2. The two phases on A1 may include a capture phase during which the data updates for R1 are captured or collected, after which the cycle transitions into a transfer phase during which the updated data for R1 is transmitted from A1 to A2 to be updated for a target LSU (R2) on A2. On A2, the cycle begins in a receive phase during which the data updates transmitted as part of the transfer phase on A1 are received on A2, and transitions to an apply phase during which the data updates are applied to R2. Replication cycles for remote replication are described in greater detail in U.S. Pat. No. 9,880,946, “Data Transfer Techniques with Data Replication,” to Benjamin Yoder et al., issued Jan. 30, 2018, the entire content of which is hereby incorporated by reference in its entirety.

In addition to employing remote replication techniques, embodiments of the invention may employ snapshot techniques, for example, as will now be described.

Referring to FIG. 7A, a replication data pointers (RDP) table 100 includes a first linked list 102 of a plurality of logical storage element (LSE) numbers 104 a-104 c, according to embodiments of the invention. A logical storage element or LSE may be any logically defined portion of an LSU, including any of: a logical data unit (as defined elsewhere herein), a track (as defined elsewhere herein), an extent or other type of portion. The RDP table 100 may be used to maintain data that is moved in connection with providing targetless snapshots, as described herein. Each of the LSE numbers 104 a-104 c may correspond to an LSE of an LSU. The LSU may be, for example, a conventional logical device with all of the LSEs having corresponding physical data storage allocated thereto or may be a thin device, as described in more detail elsewhere herein.

Each of the LSE numbers 104 a-104 c may correspond to one or more table entries that are maintained using an appropriate data structure, such as a linked list. The LSE number 104 a may correspond to a plurality of table entries 106 a-108 a, the LSE number 104 b may correspond to a plurality of table entries 106 b-108 b, and the LSE number 104 c may correspond to a plurality of table entries 106 c-108 c. Note that, although the table 100 is illustrated with three LSE numbers 104 a-104 c each having three table entries, the table 100 may contain any number of LSE numbers each having any number of table entries. In some cases, which will become apparent from the additional discussion herein, it is possible for there to be no LSE number or corresponding table entries associated with a particular LSE of an LSU. Each of the table entries 106 a-108 c may include a sequence number and a pointer to storage, which are explained in more detail elsewhere herein.

Referring to FIG. 7B, a replication data pointers (RDP) tree 110 may include a plurality of table entries 112 a-112 f that each correspond to a particular LSE, according to embodiments of the invention. Each of the table entries 112 a-112 f may include a sequence number and a pointer to storage. The RDP tree 110 may correspond to one of the linked lists pointed to by one of the data pointers 104 a-104 c of the RDP table 100, discussed above. The sequence number and the pointer to storage may be similar to the sequence number and pointer to storage used in connection with the RDP table 100, and are described in more detail elsewhere herein. In an embodiment herein, the RDP tree 110 is a balanced binary tree ordered according to sequence number.

Referring to FIG. 8, a data pool 115 may include storage for data that is moved in connection with maintaining targetless snapshots, according to embodiments of the invention. Data stored in the data pool 115 may be pointed to by the pointers provided with the table entries 106 a-108 c or the table entries 112 a-112 f In some embodiments, the data pool 115 is provided in a single logical and/or physical location. In other embodiments, the data pool 115 may be distributed and/or may use more than one physical and/or logical data storage element. Providing data to the data pool 115 is discussed in more detail elsewhere herein.

Referring to FIG. 9, a snapshot table 120 may include a plurality of entries corresponding to particular snapshots, according to embodiments of the invention. Each of the entries may include a snapshot ID and a sequence number. The snapshot ID may be used to identify a particular snapshot and could be text (e.g., “Mar. 12, 2014, 8:00 am snapshot”) or could be a token that is used by other software (not shown herein) to identify each of the snapshots. The sequence number provided with each of the snapshots may be used in connection with providing targetless snapshots and is described in more detail elsewhere herein.

Referring to FIG. 10, a sequence number table 130 is shown as having a plurality of entries, according to embodiments of the invention. Each of the entries of the table 130 may contain a sequence number, as described in more detail elsewhere herein. The table 130 may contain a single entry for each LSE number (or other appropriate data increment) of the LSU (e.g., logical device or thin device) for which targetless snapshots are being provided. Thus, for example, if there are one hundred LSEs in an LSU, there may be one hundred entries for sequence numbers in the table 130. Use of the sequence number table 130 and of sequence numbers is described in more detail elsewhere herein.

FIG. 11 is a flowchart illustrating an example of a method 1100 of performing operations in connection with performing targetless snapshots for a LSU, according to embodiments of the invention. In a step 1102, a global sequence number (associated with the LSU for which targetless snapshots are being provided) and the tables 100, 120, 130 that are used with targetless snapshots may be initialized. Note that the RDP tree 110 may be used in addition to or instead of the RDP table 100. In an embodiment herein, snapshot sequence numbers start at zero and are incremented by one for each snapshot, but of course in other instances it is possible to start at any number and increment or decrement by any amount. At the step 1102, the RDP table 100 (and/or the RDP tree 110) may be initialized to be empty (contain no entries), the snapshot table 120 may be initialized to be empty, the sequence number table 130 may be initialized so that each entry contains zero (the initial sequence number), and the global sequence number may be initialized to zero (the initial sequence number).

Following the step 1102 may be a step 1104 where the system waits for a snapshot to occur. A snapshot may be user initiated or may be automated to occur at specific times (e.g., every hour). Once a snapshot occurs, control may transfer from the step 1104 to a step 1106 where an entry corresponding to the snapshot may be created in the snapshot table 120. At the step 1106, the ID value may be provided to the new entry in the snapshot table 120 and the corresponding sequence number may be set to one greater than the current global sequence number. The ID value may include a user specified name that is to be associated with the sequence number provided to the entry. Following the step 1106 may be a step 1108 where the global sequence number is incremented. Following the step 1108, control may transfer back to the step 1104 to wait for the next snapshot to occur.

FIG. 12 is a flowchart illustrating an example of a method 1220 performed in connection with a write operation to a LSU for which snapshots are being provided, according to embodiments of the invention. In a test step 1222, it may be determined if the global sequence number equals the sequence number associated with the LSE to which the write is being provided, which may be provided by the sequence number table 130. If so, then control may transfer from the test step 1222 to a step 1224 where the write operation may be performed in a usual fashion. No special processing may be performed in this case because the global sequence number being equal to the sequence number where the data is being written means that any snapshot data associated with that particular data section has already been protected (copied to the data pool 115, as described in more detail elsewhere herein). Following the step 1224, processing may be complete.

If it is determined in the step 1222 that the global sequence number does not equal the sequence number associated with the LSE to which the write is being performed (the global sequence number is greater), then control may transfer from the step 1222 to a step 1226 where an entry in the RDP table 100 may be created by placing the new entry in a linked list using the LSE number where the write is being performed on the LSU and using the sequence number for the source LSE (from the sequence number table 130). If the RDP tree 110 is used, then in the step 1226 a new entry may be created for the RDP tree 110. Following the step 1226 may be a step 1228 where data that is being overwritten is copied from the LSU to the data pool 115. Note that the step 1228 may be omitted in instances where the LSU is a thin device and the particular LSE is empty (e.g., the pointer for the LSE points to null). Note also that, in some cases data on the LSU may be cached, in which case the copy may be from the cache memory.

Following the step 1228 is a step 1232 where the pointer in the table entry created at the step 1226, described above, may be set to point to the data in the data pool 115 that was copied at the step 1228, described above, or to null in the case of a thin logical device with no data in the LSE. Following the step 1232 is a step 1234 where the sequence number for the entry in the sequence number table 130 may be set to the global sequence number, indicating that the corresponding data written to the LSU corresponds to the current global sequence number. Following the step 1234 may be the step 1224, discussed above, where the write operation to write the new data to the device may be performed. Following the step 1224, processing may be complete.

FIG. 13 is a flowchart illustrating an example of a method 1350 of processing performed in connection with reading different versions from different snapshots of data on the LSU, according to embodiments of the invention. In a step 1352, it may be determined if a sequence number associated with a desired version (VER in flow diagram 1350) is greater than or equal to a version number from the sequence number table (SNT in the flow diagram 1350). For example, if it was desired to read a version of data associated with a particular snapshot (e.g., “8:00 am on Mar. 12, 2014”) having a sequence number X, then the test at the step 1352 may compare X with an entry in the sequence number table for the LSE of interest containing data being read, which may be provided in the sequence number table 130. If it is determined in the step 1352 that the sequence number of the desired version is greater than or equal to a version number from the sequence number table corresponding to the data being read, then data on the LSU was written prior to the time of the snapshot, and control may transfer from the step 1352 to the step 1354 where the data is read from the LSU. Note that this also may occur when it is desired to read current data from the LSU since data on the logical volume should always be the latest version. Following the step 1354, processing may be complete.

If it is determined at the step 1352 that the sequence number of the desired version is not greater than or equal to a version number from the sequence number table corresponding to the data being read, then data on the LSU was written after the time of the snapshot and the desired data is in the data pool 115, and control may transfer from the step 1352 to a step 1356 where an iteration pointer may be set to point to a first item in a list of items in the RDP table 100. The iteration pointer may be used to traverse a list of pointers for a LSE corresponding to the data being read. For the explanation herein, it may be assumed that the list of pointers is arranged with the most recently added table entry (having the highest sequence number) being first in the list, followed by the next most recently added table entry (having the second highest sequence number), etc. Generally, the iteration pointer may iterate through table entries for a specific LSE from highest sequence number to lowest sequence number. Note that, in instances where the RDP tree 110 is used, the iteration pointer may be set to point to the top of the RDP tree 110 and is used to traverse the RDP tree 110.

Following the step 1356 may be a test step 1358 where it may be determined if a sequence number associated with the desired version is greater than or equal to a version number associated with the table or tree entry indicated by the iteration pointer, similar to the test at the step 1352, discussed above. If so, then control may transfer from the test step 1358 to a step 1362 where data may be read from the data pool 115 according to the data pointer of the RDP table or RDP tree entry indicated by the iteration pointer. Following the step 1362, processing may be complete. Otherwise, if it is determined at the step 1358 that the sequence number associated with the desired version is not greater than or equal to the version number associated with the table or tree entry indicated by the iteration pointer, then control may transfer from the step 1358 to a step 1364 where the iteration pointer is set to point to a next table or tree entry. Note that the final item of the table or tree entries may have a sequence number of zero so that, eventually, the test at the step 1358 will cause the step 1362 to be executed.

In some instances, it is possible to maintain written data in memory (e.g., in a cache database in the global memory 26). Version information may be maintained with the written data in memory to facilitate eventually moving the data to the LSU while providing targetless snapshots as described herein. The data may be moved using a background process. Memory may be employed in this manner as described in the Jaganathan patent.

In some embodiments of the invention, a first LSU (R1) on a first storage system (A1), which may be considered a primary storage system, may be remotely replicated to a second LSU (R2), which may be referred to herein as a replica LSU, on a second storage system (A1), which may be considered a secondary storage system. Remote replication between R1 and R2 may become inactive after a snapshot of R1 is taken on A1, but before the replication of the snapshot to R2 is complete, resulting in an outstanding snapshot. Alternatively or additionally, one or more snapshots of R1 may be taken on A1 while remote replication between R1 and R2 is inactive, resulting in one or more outstanding snapshots. Any outstanding snapshots may be replicated to R2 when the remote replication becomes active again.

FIG. 14 is a flow diagram illustrating an example of a method 1400 of updating outstanding snapshot information for write operations received for an LSE of R1 while remote replication between R1 and R2 is inactive, according to embodiments of the invention. Other embodiments of a method of updating outstanding snapshot information for write operations received while remote replication is inactive, for example, variations of the method 1400, are possible and are intended to fall within the scope of the invention. One or more components of A1 and A2, for example, one or more of the directors 37 a-n or directors 216 a-n described in more detail in relation to FIGS. 1 and 2 may be configured to implement the method 1400.

In an initial step (not shown), it may be determined that remote replication between R1 and R2 is inactive, for example, because of a logical and/or physical failure of the communication channel between A1 and A2, or for another reason. In response to this determination, a parameter value may be defined (e.g., a replication status flag may be set) to indicate that remote replication between R1 and R2 is inactive (not shown). For example, one or more of the data structures described herein, including snapshot data structures, may include a field for defining this parameter value.

In a step 1402, a write operation for an LSE of R1 may be received on A1, for example, from a host system. It may be determined in a step 1404 whether replication between R1 and R2 is active, for example, by consulting a replication status flag corresponding to R1-R2 replication. If the remote replication is active, normal remote replication and snapshot processing may be performed in accordance with however A1 is configured.

If it determined in the step 1404 that remote replication between R1 and R2 is not active, i.e., is inactive, then in a step 1406 it may be determined whether the LSE is already marked as replication dirty, meaning that there has already been at least one write operation for the LSE that has not been replicated yet to R2, for example, because it was received during the period in which R1-R2 remote replication is inactive. If it is determined in the step 1406 that the LSE is not already marked as replication dirty, then the LSE may be marked as replication dirty in a step 1408. In some embodiments, as an alternative to performing the step 1406, the LSE may be marked as replication dirty regardless of whether it is already so marked.

After performance of the step 1408, or if it is determined in the step 1406 that the LSE is already marked as replication dirty, then in a step 1410 it may be determined whether the write operation received in the step 1402 is a first write operation since a last snapshot of the LSU was taken. For example, in some embodiments this may be achieved by determining the snapshot sequence number associated with the LSE (e.g., from table 130), and comparing the LSE snapshot sequence number with the (global) snapshot sequence number of the LSU. If the snapshot sequence numbers are the same, this means that a write operation has already been made to LSE since the last snapshot was taken, i.e., the write operation received in the step 1402 is not the first write operation since the last snapshot. If the snapshot sequence numbers are different (i.e., the LSU sequence number is greater than the LSE sequence number), this means that the write operation received in the step 1402 is the first write operation since the last snapshot.

If it is determined in the step 1410 that the write operation received in the step 1402 is the first write operation since a last snapshot of the LSU was taken, then snapshot metadata may be updated (e.g., marked) to reflect that the LSE is dirty for the last snapshot. Otherwise, if the write operation is not the first write operation since the last snapshot, the snapshot metadata should already be updated to reflect that the LSE is dirty for the last snapshot. In some embodiments, as an alternative to performing the step 1410, the snapshot metadata may be updated to reflect that the LSE is dirty for the last snapshot regardless of whether it has already been so updated.

After performance of the step 1412, or if it is determined in the step 1410 that the write operation is not the first write operation since a last snapshot of the LSU, other snapshot processing may be performed on R1, for example, as described in more detail herein or in Jaganathan and/or Tobin.

It should be appreciated that, concurrently to the performance of the steps 1404-1414, other processing of the write operation besides synchronization and snapshot processing may be performed by A1.

In some embodiments, each write operation received for R1 may be initially marked as outstanding, and this mark is removed (e.g., the value of a parameter is changed) to reflect that the write operation is not outstanding when replication of the write to R2 is completed. In this manner, a write operation is outstanding not only if it is performed on R1 while R1-R2 replication is inactive, but also if it was performed on R1 prior to R1-R2 becoming inactive but did not complete replicating (or perhaps even start replicating) before R1-R2 became inactive.

In addition, for any snapshot taken of R1 while R1-R2 replication is inactive, the snapshot may be marked as outstanding, for example, in one or more of the snapshot metadata structures described in herein. In some embodiments, each snapshot may initially be marked as outstanding, and this mark is removed (e.g., the value of a parameter is changed) to reflect that the snapshot is not outstanding when replication of the snapshot (by whatever process is being employed) is completed. In this manner, a snapshot is outstanding not only if it is taken while R1-R2 replication is inactive, but also if it was taken prior to R1-R2 becoming inactive but did not complete replicating (or perhaps even start replicating) before R1-R2 became inactive. It should be appreciated that the replication of a snapshot includes replicating any snapshot data created for the snapshot (i.e., as a result of a write operation) and any snapshot metadata modified as a result of the created snapshot data.

In some embodiments, for each LSU, a list of outstanding snapshots for the LSU may be maintained, and/or a master list of outstanding snapshots for all LSUs may be maintained. After remote replication is re-established between R1 and R2, the one or more outstanding snapshot lists and outstanding snapshot information recorded during performance of the method 1400 may be applied to remotely replicate the outstanding snapshots.

FIGS. 15A and 15B present a flow diagram illustrating an example of a method 1500 of remotely replicating outstanding snapshots, according to embodiments of the invention. Other embodiments of a method of remotely replicating outstanding snapshots, for example, variations of the method 1500, are possible and are intended to fall within the scope of the invention. The method 1500 may be controlled by a process, P, on A1 separate from the one or more processes on A1 that are executing standard remote replication and/or copying outstanding write operations from the inactive replication period from R1 to R2. In some embodiments, the process P may be given low priority on A1 so as to not impair performance of other operations (including standard remote replication and copying outstanding write operations) on A1. For example, the process P may be run as a background process on A1. One or more components of A1 and A2, for example, one or more of the directors 37 a-n or directors 216 a-n described in more detail in relation to FIGS. 1 and 2 may be configured to implement the method 1500, including the process P.

In the following description, a snapshot of R1 may be referred to herein as SSn₁, where n is an ID (e.g., number) of the snapshot, and the number “1” indicates that the snapshot is of R1. A snapshot of R2 that is a replica of a snapshot of R1 may be referred to herein as SSn₂, where n is an ID (e.g., number) of the snapshot being replicated, and the number “2” indicates that the snapshot is of R2. For example, a first outstanding snapshot of R1 may be referred to herein as SS1₁, and a replica snapshot of the first outstanding snapshot, a snapshot of R2, may be referred to herein as SS1₂; and a second outstanding snapshot of R1 may be referred to herein as SS2₁, and a replica snapshot of the second outstanding snapshot, a snapshot of R2, may be referred to herein as SS2₂, and so on.

In a step 1502, a first outstanding snapshot (e.g., the earliest outstanding snapshot) of R1, SS1₁, may be accessed on A1, for example, from a master outstanding snapshot list or an outstanding snapshot list specific to R1.

In a step 1504, a snapshot replica of SS1₁, namely snapshot SS1₂ of R2, may be initialized (i.e., seeded) on A2, e.g., in response to an instruction from A1. For example, SS1₂ may be created (e.g., an ID (e.g., a number) and description may be specified for SS1₂, and a snapshot sequence number associated with SS1₂), and the metadata for each LSE of SS1₂ configured to point to the current data for R2. For example, the snapshot sequence number for each LSE may be set to one less than a snapshot sequence number of R1, e.g., as described in more detail herein, in Jaganathan and/or in Tobin. In some embodiments, while SS1₂ is being initialized, standard remote replication between R1 and R2 and the copying of outstanding write operations made during the inactivity of R1-R2 replication may be suspended, so that the current state of R2 reflects the state of R2 when the inactivity period began, so the initially seeded SS1₂ accurately reflects the state of R2 when the inactivity period began.

In a step 1506, a current LSE may be set to a first LSE of R1, and, in a step 1508, it may be determined whether the current LSE is replication dirty; i.e., whether a last (i.e., most recent) write operation (if any) made for the LSE is still outstanding—i.e., has not been copied to R2 yet. If so, the method 1500 may wait until the last write operation is copied as reflected by the arrowed line for “Yes” returning to step 1508 itself. Alternatively, the method 1500 may proceed to the next LSE and perform the alternative iterative process described in more detail elsewhere herein.

If it is determined in the step 1508 that the LSE is not replication dirty, then in a step 1510 it may be determined whether the value of the LSE for SS1₁ is the current value of the LSE. For example, it may be determined whether the snapshot metadata of the LSE for SS1₁ points to the same data pointed to by the metadata for the current value of the LSE. The snapshot metadata of the LSE may include the sequence number table 130, RDP table 100 and/or RDP tree 110 described elsewhere herein or in Jaganathan or the snapshot look-up tables (SLTs), current look-up tables (CLTs) and snapshot pointer structures (SPSs) described in Tobin. The metadata for the current value of the LSE may include the master LSU table 62, the LSU table 72, the thin device table 72′ and the LSU track table 82 described in more detail elsewhere herein.

If the value of LSE for SS1₁ is not the current value of the LSE, then at least one write operation for the LSE has been performed since the first outstanding snapshot SS1₁ was taken (i.e., since the point in time represented by SS1₁), and the method 1500 may proceed to a step 1512. In the step 1512 it may be determined whether the snapshot data for the LSE for SS1₁ is dirty, meaning that at least one write operation was made to the LSE since SS1₁ was taken and the resulting snapshot data and corresponding snapshot metadata changes for SS1₁ have not yet been replicated to R2.

If it is determined in the step 1512 that the snapshot data for the LSE for SS1₁ is dirty, then, in a step 1514, the LSE SS1₁ snapshot data may be copied from A1 to A2 to be applied to replica snapshot SS1₂ of R2. For example, a special command may be sent from A1 to A2 to indicate that this copy is outside of standard replication procedure, and is to be handled differently than a standard replication command. A2 may be configured to recognize the special copy command.

In a step 1516, in response to receiving the special copy command, A2 may allocate a storage location (e.g., from a pool of storage reserved for snapshot data) for the copied snapshot data, and the copied snapshot data may be stored at the allocated location in a step 1517. In a step 1518, the LSE SS1₂ metadata may be modified to point to the allocated location, for example, as described herein, in Jaganathan and/or in Tobin.

In a step 1520, A1 may mark the LSE SS1₁ data as clean, after which the method 1500 may return to the step 1522. The step 1522 also may be reached: if it determined in the step 1510 that the value of the LSE for SS1₁ is the current value of the LSE, which means that no write operations were made to LSE since SS1₁ was taken and thus there is no snapshot data to be copied to R2 for the LSE; or if it is determined in the step 1512 that the snapshot data for the LSE for SS1₁ is not dirty.

In the step 1522, it may be determined whether there is a next LSE of R1 to process. If there is a next LSE of R1 to process, the method 1500 may return to the step 1509; otherwise, in a step 1524 it may be determined whether there is a next outstanding snapshot for R1. If there is a not next outstanding snapshot for R1, the method 1500 may end; otherwise the method may proceed to the step 1526 for a next outstanding snapshot SS2₁ on R1.

In some embodiments, the steps 1502-1524 illustrated in FIG. 15A may be performed for the first outstanding snapshot of R1, whereas the steps 1526-1541 may be performed for the remaining outstanding snapshots of R1.

In a step 1526, A2 may initiate (seed) a snapshot replica SSn₂ (e.g., SS2₂ on a first pass) of SSn₁ (e.g., SS2₁ on a first pass) by cloning a last (i.e., immediately preceding) snapshot replica for R2, SSn-12 (e.g., SS1₂ on a first pass). For example, SS2₂ may be created, and the metadata for each LSE of SS2₂ configured to point to same data pointed to by the snapshot metadata of SS1₂.

In a step 1528, A1 may determine which LSEs of R1 (if any) have had data changed between SSn-11 and SSn₁; i.e., which LSEs have had their data modified by one or more write operations between the snapshots. Any of a variety of known algorithms for determining differences between snapshots may be used, for example, using any of the snapshot metadata described in more detail elsewhere herein, in Jaganathan and/or in Tobin. Only the LSEs having modified data may be analyzed to determine whether LSE SSn₁ data has not been replicated to R2.

In a step 1532, it may be determined whether there is a next LSE of R1 for which data has changed since the previous snapshot SSn−1. If it is determined in the step 1532 that there is not a next LSE of R1 for which data has changed since the previous snapshot SSn−1, then processing may be considered complete for snapshot SSn₁, and in a step 1530 it may be determined whether there is a next outstanding snapshot for R1. If there is not a next outstanding snapshot, the method 500 may end; otherwise the method may return to the step 1526 for the next outstanding snapshot of R1.

If it is determined in the step 1532 that there is a next LSE of R1, then in a step 1535 it may be determined whether the LSE value for SSn₁ is the current value for the LSE. For example, it may be determined whether the snapshot metadata of the LSE for SSn₁ points to the same data pointed to by the metadata for the current value of the LSE. If the LSE value for SSn₁ is the current value for the LSE this means that there have not been any write operations performed for the LSE since SSn₁ was taken (i.e., since the point in time represented by SSn₁); otherwise there has been at least one write operation since for the LSE since SSn₁ was taken.

If it is determined in the step 1535 that the LSE value for SSn₁ is the current value for the LSE, then, in a step 1536, A1 may send an instruction to R2 to update SSn₂ metadata to point to the current value of the LSE. The SSn₂ metadata may be implemented as described herein, in Jaganathan or Tobin. The current value of the LSE may be determined by accessing metadata for the current value of the LSE, for example, such metadata described in more detail elsewhere herein. In a step 1538, A2 may update the SSn₂ metadata in response to receiving the instruction from A1.

Prior to performing the step 1536, A1 may lock access to a memory location (e.g., a cache slot) at which the current value of the LSE is stored in a step 1536, to prevent other processes on A1 from potentially modifying a value of the LSE while the steps 1536 and 1538 are being performed, and this lock may be released in a step 1542 after the steps 1536 and 1538 have been performed. The memory location may be determined, for example, by accessing an entry in the cache slot table 300 for the LSE.

If it is determined in the step 1535 that the LSE value for SSn₁ is not the current value for the LSE, “taking

Various embodiments of the invention may be combined with each other in appropriate combinations. Additionally, in some instances, the order of steps in the flowcharts, flow diagrams and/or described flow processing may be modified, where appropriate. It should be appreciated that any of the methods described herein, including methods described in relation to FIGS. 11-15B, or parts thereof, may be implemented using one or more of the systems and/or data structures described in relation to FIGS. 1-10, or components thereof. Further, various aspects of the invention may be implemented using software, firmware, hardware, any suitable combination thereof and/or other computer-implemented modules or devices having the described features and performing the described functions. Logic that when executed performs methods described herein, steps thereof or portions of such methods or steps, may be implemented as software, firmware, hardware, or any suitable combination thereof.

Software implementations of embodiments of the invention may include executable code that is stored on one or more computer-readable media and executed by one or more processors. Each of the computer-readable media may be non-transitory and include a computer hard drive, ROM, RAM, flash memory, portable computer storage media such as a CD-ROM, a DVD-ROM, a flash drive, an SD card and/or other drive with, for example, a universal serial bus (USB) interface, and/or any other appropriate tangible or non-transitory computer-readable medium or computer memory on which executable code may be stored and executed by a processor. Embodiments of the invention may be used in connection with any appropriate OS.

As used herein, an element or operation recited in the singular and preceded with the word “a” or “an” should be understood as not excluding plural elements or operations, unless such exclusion is explicitly recited. References to “one” embodiment or implementation of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Furthermore, a description or recitation in the general form of “at least one of [a], [b] or [c],” or equivalent thereof, should be generally construed to include [a] alone, [b] alone, [c] alone, or any combination of [a], [b] and [c]. In addition, use of a an ordinal term, e.g., “first,” “second” or the like, to qualify a term for an item having multiple instances of the same name does not necessarily indicate a priority, precedence or temporal order between the instances unless otherwise indicated, but rather such ordinal terms may be used merely to distinguish between the separate instances.

Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. For a system including a first storage system, a second storage system, and a first logical storage unit, wherein the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system, a method comprising: recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system, wherein the replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit.
 2. The method of claim 1, wherein the replicating of the plurality of snapshots is performed as part of a first process that is executed independently of one or more processes executing the replicating of the write operations.
 3. The method of claim 1, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, including, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system.
 4. The method of claim 3, wherein the replicating of the first snapshot to the first replica snapshot includes, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system.
 5. The method of claim 1, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, including: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot.
 6. The method of claim 5, further comprising, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system.
 7. The method of claim 5, further comprising, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data.
 8. A system comprising: a first storage system; a second storage system; a first logical storage unit, wherein the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system; and executable logic that implements a method including: recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system, wherein the replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit.
 9. The system of claim 8, wherein the replicating of the plurality of snapshots is performed as part of a first process that is executed independently of one or more processes executing the replicating of the write operations.
 10. The system of claim 8, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, including, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system.
 11. The system of claim 10, wherein the replicating of the first snapshot to the first replica snapshot includes, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system.
 12. The system of claim 8, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, including: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot.
 13. The system of claim 12, wherein the method further includes, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system.
 14. The system of claim 12, wherein the method further includes, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data.
 15. For a system including a first storage system, a second storage system, and a first logical storage unit, wherein the first storage system is configured to replicate data of the first logical storage unit to a second logical storage unit on the second storage system, computer-readable media having software stored thereon comprising: executable code that controls, recording a list of a plurality of snapshots of the first logical storage unit that were taken on the first storage system but not replicated to the second storage system because of a first period of time during which the replication of the first logical storage unit to the second logical storage unit is inactive; and executable code that controls, in response to the replication of the first logical storage unit to the second logical storage unit becoming active, accessing the list and replicating the plurality of snapshots of the list from the first storage system to replica snapshots on the second storage system, wherein the replicating of the plurality of snapshots is performed concurrently to replicating write operations as part of the replication of the first logical storage unit to the second logical storage unit.
 16. The computer-readable media of claim 15, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: replicating a first snapshot of the plurality of snapshots to a first replica snapshot on the second storage system, including, for each logical storage element of the first logical storage unit: determining if one or more write operations received on the first storage system for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit on the second storage system; and if the one or more write operations for the logical storage element have had data stored on the first storage system but not yet replicated to the second logical storage unit, waiting for the data of the one or more write operations to be replicated to the second logical storage unit before replicating snapshot data of the logical storage element from the first storage system to the first replica snapshot on the second storage system.
 17. The computer-readable media of claim 16, wherein the replicating of the first snapshot to the first replica snapshot includes, for each logical storage element of the first logical storage unit: determining if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system; and if any snapshot data for the logical storage element has been stored on the first storage system, but not yet replicated to the second storage system, replicating the snapshot data from first storage system to the second storage system.
 18. The computer-readable media of claim 15, wherein the plurality of snapshots are listed in the list in a chronological order from an earliest point in time corresponding to a snapshot to a latest point in time corresponding to a snapshot, and wherein the replicating of the plurality of snapshots includes: after replicating a first snapshot of the plurality of snapshots, replicating a second snapshot of the plurality of snapshots to a second replica snapshot on the second storage system, including: for each logical storage element of the first logical storage unit, determining if the logical storage element data has been modified between the first snapshot and the second snapshot; and performing the remaining processing for replicating the second snapshot snapshots to the second replica snapshot on only the logical storage elements of the first logical storage unit that have been modified between the first snapshot and the second snapshot.
 19. The computer-readable media of claim 18, wherein the software further comprises, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: executable code that controls determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and executable code that controls, if the value of the logical storage element for the second snapshot is the same value as the current value of the logical storage element on the first storage system, updating metadata of the second replica snapshot for the logical storage element to reference the current value of the logical storage element on the second storage system.
 20. The computer-readable media of claim 18, wherein the software further comprises, for each logical storage element of the first logical storage unit that has been modified between the first snapshot and the second snapshot: executable code that controls determining if a value of the logical storage element for the second snapshot is a same value as a current value of the logical storage element on the first storage system; and executable code that controls, if the value of the logical storage element for the second snapshot is not the same value as the current value of the logical storage element on the first storage system: copying snapshot data of the logical storage element specified for the second snapshot from the first storage system to the second storage system; and updating metadata of the second replica snapshot for the logical storage element to reference the copied snapshot data. 